The present invention relates to a semiconductor laser device capable of emitting a plurality of laser beams having different wavelengths.
The present application claims priority from Japanese Application No. 2002-374636, the disclosure of which is incorporated herein by reference.
In recent years, development and research have been carried out on semiconductor laser device called multiple wavelength laser which is capable of emitting a plurality of laser beams having different wavelengths.
For example, in the field of data recording and playback systems for optical storage medium represented by CD (Compact Disc) and DVD (Digital Versatile Disc), if it is desired to develop an optical pickup compatible with various storage media, it is important to develop a semiconductor laser device capable of emitting a plurality of laser beams having different wavelengths.
In order to solve some difficulties in realizing the multiple-wavelength laser diode produced by monolithic integration technique, it has been suggested that a hybrid structure be used to achieve the same purpose (for example, patent document 1).
A semiconductor laser device disclosed in patent document 1, as shown in FIG. 1 of the patent document, comprises a first light emitting element which is formed on a first substrate and has a GaN-based laser oscillation section capable of emitting a short wavelength (for example, about 400 nm) laser beam, and a second light emitting element which is formed on a second substrate and has an AlGaInP-based laser oscillation section and an AlGaAs-based laser oscillation section capable of emitting a long wavelength (for example, from about 600 nm to about 700 nm) laser beam. The first and second light emitting elements are fabricated independently from each other and in the form of chips, with one being laid above the other on the support substrate (so called sub-mount), thereby forming a hybrid structure.
Here, the first light emitting element is attached to the support substrate and the second light emitting element is attached to the first light emitting element.
In detail, since the GaN-based laser oscillation section provided on the first substrate is attached such that it is interposed between the first substrate and the support substrate, the first light emitting element can be attached to the support substrate. Further, since an AlGaInP-based laser oscillation section and an AlGaAs-based laser oscillation section provided on the second substrate are attached such that they are interposed between the second substrate and the first substrate, the second light emitting element can be attached to the first substrate.
Namely, the GaN-based laser oscillation section, the first substrate, the AlGaInP-based laser oscillation section, the AlGaAs-based laser oscillation section and the second substrate are successsively laid in such an order on the support substrate, thereby forming a multilayer structure.
The patent document 1 shows that a pickup which has a compatibility with various storage media can be realized if such semiconductor laser device is used as a light source for optical pickup.
However, since the above-described conventional semiconductor laser device is so formed that the first light emitting element and the second light emitting element are fabricated in advance as independent semiconductor chips, and since the first and second light emitting elements in the form of chips are stacked one over another on the support substrate (sub-mount) and thus an overlapped structure is formed, there have been existing the following problems.
Namely, when manufacturing a semiconductor laser device for use in an optical pickup, in order to adjust the emission direction of a laser beam emitted from a cleaved facet of each light emitting element, it is necessary to attach each chipped light emitting element by positioning the same with an extremely high precision, hence requiring an extremely complex process of manufacturing respective semiconductor laser devices.
Furthermore, in the conventional semiconductor laser device, the first light emitting element has a structure in which GaN-based laser oscillation section is formed on the first substrate, while the second light emitting element has a structure in which AlGaInP-based laser oscillation section and AlGaAs-based laser oscillation section are formed on the second substrate. Then, since GaN-based laser oscillation section is attached to the support substrate, the first substrate is positioned on the upper side of GaN-based laser oscillation section, while AlGaInP-based laser oscillation section and AlGaAs-based laser oscillation section are positioned on the upper side of the first substrate, while the second substrate is positioned on the upper sides of the AlGaInP-based laser oscillation section and AlGaAs-based laser oscillation section.
Namely, the first substrate is disposed between the GaN-based laser oscillation section on one side and the AlGaInP-based laser oscillation section as well as AlGaAs-based laser oscillation section on the other.
However, as described in the above patent document 1, in the aforementioned structure, since the first substrate (GaN substrate) usually has a thickness of 100 μm, there is a problem that a large interval is formed between an emission position (light emitting spot) of laser beam of GaN-based laser oscillation section and another emission position (light emitting spot) of laser beams of the AlGaInP-based laser oscillation section and AlGaAs-based laser oscillation section. Namely, there is a problem that an interval between the respective light emitting spots of the respective laser beams is large.
For example, when this semiconductor laser device is mounted on an optical pickup to perform information recording or reproducing, if an emission position (light emitting spot) of GaN-based laser oscillation section is aligned in its optical axis with the optical axis of an optical system forming the optical pickup, emission positions (light emitting spots) of the AlGaInP-based laser oscillation section and AlGaAs-based laser oscillation section will greatly deviate from the optical axis center of the optical system due to an influence of the thickness of the first substrate, hence causing some troubles such as aberration.
Moreover, for example, if an optical element such as a prism is provided for eliminating some unfavorable influences caused by the thickness of the first substrate in order that laser beam emitted by the GaN-based laser oscillation section as well as the laser beams emitted by the AlGaInP-based laser oscillation section and the AlGaAs-based laser oscillation section may all become coincident with the optical axis of the optical system of the optical pickup, another problem will occur that the number of parts will have to be increased.